[hotfix] Use default value of RocksDBOptions#TIMER_SERVICE_IMPL

In order to fully enable the RocksDBOptions#TIMER_SERVICE_IMPL we should use the default value
of this config option instead of "".

flink-state-backends/flink-statebackend-rocksdb/src/main/java/org/apache/flink/contrib/streaming/state/RocksDBStateBackend.java

@@ -271,7 +271,7 @@ public class RocksDBStateBackend extends AbstractStateBackend implements Configu
 		this.enableIncrementalCheckpointing = original.enableIncrementalCheckpointing.resolveUndefined(
 			config.getBoolean(CheckpointingOptions.INCREMENTAL_CHECKPOINTS));
 
-		final String priorityQueueTypeString = config.getString(TIMER_SERVICE_IMPL.key(), "");
+		final String priorityQueueTypeString = config.getString(TIMER_SERVICE_IMPL);
 
 		this.priorityQueueStateType = priorityQueueTypeString.length() > 0 ?
 			PriorityQueueStateType.valueOf(priorityQueueTypeString.toUpperCase()) : original.priorityQueueStateType;